Known joint forming sheets for electrical connection include a connecting sheet comprising a substrate with a easy release surface, such as a fluorine resin substrate, having provided thereon a low-melting metallic material layer as disclosed in JP-A-64-10578 and JP-A-64-10579 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and a connecting sheet comprising a substrate made of plastics other than fluorine resins having provided thereon a low-melting metallic material layer as disclosed in JP-A-64-63205.
An electrical joint can be formed by bringing the low-melting metallic material layer of a connecting sheet into contact with a part on which a joint is to be formed and pressing the layer from the back of the substrate with or without heat to thereby cut the pressed part of the metallic material layer and then transfer the cut part onto the part to be connected. Use of such a connecting sheet improves efficiency of electrical connection of various electrical or electronic parts.
However, where a metallic material layer is provided on an easy release surface of, for example, a fluorine resin substrate merely aiming at easy removal from the substrate, the pressed part of the metallic material layer tends to be released from the substrate before being cut. As a result, the transferred metallic layer would be oversized with poor dimensional precision. Such poor precision of transfer causes a failure of electrical connection.
On the other hand, where a metallic material layer is provided on a surface of a substrate made of plastics other than fluorine resins, since the metallic material layer is not easily released from the substrate at the adhesive interface, it tends to be broken in parts before being released, resulting in insufficient transfer. If the tensile strength of the metallic material layer is increased to avoid this, then the metallic material layer cannot be smoothly transferred, resulting in a reduction of workability.
Thus, the conventional techniques of joint formation by use of a connecting sheet have not yet achieved sufficient precision of transfer, waiting for further improvements.